1. Technical Field
The present invention relates generally to an improved communications system and in particular to a method and apparatus for tracking mobile stations within a wireless communications system.
2. Description of the Related Art
Many of the present digital telecommunications networks of the world are based on time-division multiple access (TDMA) and use transmission and switching products like digital switches and pulse coded modulation (PCM) transmission at E1 or T1 rates. TDMA users share the radio spectrum in the time domain. An individual user is allocated a time slot and, during this time slot, the user accesses the whole frequency band allocated to the system (wideband TDMA) or only a part of the band (narrow band TDMA). In TDMA, transmission takes place in bursts from a mobile station to a base station in an uplink path with only one user transmitting to the base station at any given time. In the downlink path from the base station to the mobile station, the base station usually transmits continuously with the mobile station listening only during the assigned time slot. TDMA channel multiplexes the bits from a number of users. This type of system requires transmission at a higher bit rate over a radio frequency channel.
Another technology that is becoming more widely used is code-division multiple access (CDMA) or spread spectrum. Spread spectrum techniques spread the bandwidth of the transmitting signal over a spectrum or band of frequency much larger than the minimal bandwidth required to transmit the signal. CDMA has an ability to lock out conflicting signals, which may allow it to share a system with other radio signals without interference.
Wireless technology is an important component of the global information infrastructure. To exploit the full potential of this technology, regulators and administrators of all countries are re-evaluating their spectrum allocation policy. A large segment of the spectrum around two GHz bandwidths has already been released for the use of wireless networks. More spectrum is being allocated by the International Telecommunications Union (ITU) and countries like the United States of America and Canada around the 7-10 GHz range to use wireless technology for broadband wireless applications. Wireless networks are being built all over the world to handle a large volume of traffic and mobility.
With respect to tracking and locating mobile stations within wireless communications systems, the current systems employ a location area based paging and location update scheme. FIG. 1 illustrates one scheme used in current wireless communications systems. In FIG. 1, mobile station 100 reports its location to the network whenever it enters a new registration area, also called a "location area". Seven location areas, identified as LA1 through LA7, are illustrated in FIG. 1 with each location area having seven cells. Mobile station 100 travels a path represented by dotted line 102, beginning at location area LA3, continuing to location area LA6, into location area LA4, and traveling back to location area LA6. Then mobile station 100 travels through location area LA7, returning to location area LA4, traveling through location area LA5, and mobile station 100 stopping in location area LA4.
When an incoming call arrives for mobile station 100, all cells in the residing location area LA4 page to determine the residing cell of mobile station 100. Using this method, the location areas are fixed. They cannot be adjusted from time to time based on the mobility and calling patterns of each mobile station. Furthermore, changing the location areas in the current system requires modifications to the system configuration. Different mobile stations cannot be assigned different sets of location areas even though the mobility and calling patterns vary widely among mobile stations.
Other schemes used to track and locate mobile stations include per-user based location management schemes such as time-based, distance-based and movement-based location update schemes. Under the time-based scheme, a mobile station reports its location to the network when a specified time period has elapsed after its last location update. This scheme allows the dynamic selection of the location update time interval on a per-user basis. However, a user may be located at or close to the last updated cell location when the time-out occurs. This results in unnecessary location updates. Besides, this scheme does not allow the system to determine a specific area where the mobile station can be found.
In the dynamic time-based location update scheme, the location update time interval is determined based on the probability distribution of the call arrival time. This scheme does not require any information on the mobility pattern of the subscriber. It has been demonstrated that the performance of this scheme is close to that of a distance-based scheme. However, the paging delay for this scheme is not confined and is proportional to the distance travelled by the subscriber since the last location update. This scheme may not be appropriate for systems with strict connection setup delay requirements.
Under the distance-based scheme, a mobile station reports its location to the network when it determines its distance from its last update cell location exceeds a threshold value. This method performs well since location reporting is necessary only when the mobile station is far away from its last updated location. Implementation of this scheme is complicated as it is difficult for the mobile station to determine its distance from its last updated cell location.
Under the movement-based scheme, a mobile station reports its location to the network after it has detected a specified number of cell boundary crossings. This method is simple to implement as a mobile station only has to keep a movement counter and reports its location to the network when the counter exceeds a threshold value. This method, however, does not perform as well as the distance based scheme as the mobile station may be close to or at the last updated location even though it has performed a specified number of cell boundary crossings.
The above three schemes allow dynamic adjustments on a per user basis. However, not much has been reported on how to implement these schemes under the current cellular network infrastructure. All of the above schemes require some knowledge of the mobility and calling characteristic of the mobile stations, but it is unclear how these parameters can be obtained. Therefore, it would be desirable to have an improved method and apparatus for dynamically tracking mobile stations in a wireless communications system.